Irony is a contrast between appearance and actuality. In this story, the innocent young boy thinks he's in love with Mangan's sister. He sees himself as a religious hero and equates the young girl to the Virgin Mary. His feelings for her at the beginnning are very strong. "But my body was like a harp and her words and gestures were like fingers running upon the wires." He confuses his crush on her with his religious teachings and goes to Araby, a bazaar sponsored by the church, to buy her a present when the young girl says she can't go. He tells her he'll buy her something if he goes to the bazaar.
The young narrator expects the bazaar to be magical, a place that reflects his feelings for the girl and his enthusiasm for his religion. He has an "epiphany" after he arrives, a moment of insight where he understands the actuality of his feelings for the young girl. Once he understands his feelings for her, he then sees Araby for what it is, just a place to buy trinkets. He feels angry at himself and probably is disappointed, but this is part of his transformation from innocence to a more mature understanding of what it's all about. He expects one thing before he gets to Araby, but his "epiphany" allows him to see the reality of what it really is.